This application claims the priority of German Patent Application Serial No. 101 20 026.9, filed Apr. 24, 2001, pursuant to 35 U.S.C. 119(a)-(d), the subject matter of which is incorporated herein by reference.
The present invention relates, in general, to a pressure fluid cylinder, and more particularly to a pressure fluid cylinder of a type having a cylinder tube of non-circular inner cross section and a piston received in the tube and having one end connected to a piston rod in coaxial relationship.
Pressure fluid cylinders of this type have been widely used, in particular in automation systems, in order to produce a linear adjustment force through application of a fluid pressure upon the piston, whereby the pressure fluid may be a hydraulic liquid or compressed air.
German Pat. No. DE 24 31 706 A1 discloses a pressure fluid cylinder with a cylinder tube which has a polygonal inner cross section for interaction with a complementary piston so as to provide an anti-rotation structure for a piston rod connected to the piston. The polygonal configuration thus prevents the piston rod from rotating, when the piston rod is acted upon by a moment.
It is also known to attain an anti-rotation structure by designing the piston rod with a non-circular cross section, whereby the piston rod interacts via a suitable sealing arrangement with a complementary opening in the cylinder lid of the pressure fluid cylinder to prevent rotation. The cylinder tube as well as the inner piston can then have a conventional circular configuration, while the piston rod has normally a polygonal or elliptic cross section.
Another conventional approach involves the provision of two piston rods of circular cross section which are secured to the piston in parallel spaced-apart relationship and guided through respective openings in the cylinder lid, to realize an anti-rotation structure. Also in this case, the cylinder tube as well as the inner piston can have a circular cross section.
Manufacturers oftentimes market pressure fluid cylinders in a version with anti-rotation feature and in a version without anti-rotation feature to suit customer""s demands. To make both these versions commercially available, version-specific components are needed. Typically, both the cylinder tube and the complementary piston have a non-circular configuration in the pressure fluid cylinder version with anti-rotation feature, while the pressure fluid cylinder version without anti-rotation feature has both the cylinder tube and the complementary piston with circular configuration. Keeping separate constructions is cost-intensive, also when taking into account that sealing elements and other components must be tailored as well for use in either of these versions.
It would therefore be desirable and advantageous to provide an improved pressure fluid cylinder which obviates prior art shortcomings and which is constructed to keep the number of components to a minimum while useable with or without integrated an anti-rotation feature.
According to one aspect of the present invention, a pressure fluid cylinder includes a cylinder tube with non-circular inner cross section, a piston sealingly received in the tube and having a circular cross section, a piston rod connected in coaxial relationship to one end of the piston, an annular adapter connected to the piston in surrounding relationship and having a circular inner surface area to complement a circular outer surface area of the piston so as to permit a relative rotation between the piston and the adapter, said adapter having a non-circular outer surface area to complement the non-circular inner cross section of the tube, and a fastening unit provided to realize a locked engagement of the adapter to the piston.
The present invention resolves prior art problems by disposing an annular adapter between the piston and the cylinder tube, wherein the adapter has a circular inner surface area in correspondence with the circular cross section of the piston, and a non-circular outer surface area in correspondence to the non-circular inner wall surface of the cylinder tube. In this way, a same pressure fluid cylinder can be configured with or without anti-rotation feature and can be offered commercially as a kit to satisfy both versions if need be. A conversion of the pressure fluid cylinder from the version without anti-rotation feature to the other version with anti-rotation feature can be realized by simply securing the adapter to the piston by means of the fastening unit so to prevent a relative movement therebetween and as a consequence of the complementary non-circular inner configuration of the cylinder tube and the adapter, whereby the adapter is firmly seated on the piston. To reverse the anti-rotation feature and thus to convert the pressure fluid cylinder to the version without anti-rotation feature, the fastening unit is simply removed. As a result, the number of individual components is significantly reduced, while still making available both versions.
According to another feature of the present invention, the fastening unit includes at least one pin insertable in aligned radial boreholes in the piston and the adapter to prevent a relative rotation between the piston and the adapter. The provision of two pins is, however, currently preferred, whereby one pin is insertable in aligned radial boreholes in the piston and the adapter at one location, and the other pin is insertable in aligned radial boreholes in the piston and the adapter at another location which is spaced from the one location. This configuration establishes an optimal compromise between integrity of the anti-rotation feature with regard to a moment acting on the piston rod, on the one hand, and the intent to keep the number of components to a minimum, on the other hand.
As an alternative, the fastening unit may be implemented by a screw, e.g. a set screw, for threaded engagement in a threaded bore of the piston to clamp the adapter. The threaded bore may also be provided in analogous manner in the adapter. Of course, any other configurations or fastening units that are appropriate and known to the artisan to establish a form-fitting or forced engagement may be applicable as well, without departing from the spirit of the present invention.
According to another feature of the present invention, the piston may have a groove-shaped outer radial recess for so receiving the adapter in a form-fitting manner as to allow relative rotation between the piston and the adapter. The form-fitting connection of both components is easy to make and results in a desired rotatablity of the adapter, while effectively inhibiting a relative axial movement. Suitably, the adapter is made of two shells to facilitate the installation of the adapter into the recess, whereby the partition line between the shells is provided in the area of the recess. As an alternative, the adapter may also be composed of two parts.
The adapter may be made of plastic and fabricated through an injection molding process. During fabrication, at least one permanent magnet can be embedded in the adapter to monitor the piston position in conjunction with an inductive sensor placed externally on the cylinder tube. There may also be provided at least one fastening tab, formed externally on the tube and extending in parallel relationship to the cylinder axis, for detachably securing such a sensor or a valve for supply of fluid to the pressure fluid cylinder. A tab-like configuration of the attachment is beneficial compared to a grooved configuration because contaminants cannot deposit as easily. This benefit is in particular of relevance, when a pressure fluid cylinder according to the present invention is used in the food industry which has to observe stringent hygienic regulations.
According to another feature of the present invention, there is provided at least one sealing element which has a substantially ring-shaped configuration and is disposed in a space between the piston and an inner wall of the tube. The sealing element is hereby so held in the space upon the piston to allow rotation thereto, and interacts with the inner wall of the tube for dynamically sealing the inner wall so as to have an outer contour which matches the non-circular cross section. On the inside, the sealing element has a circular configuration to allow a rotation relative to the piston. Currently preferred is an arrangement of exactly two ring-shaped sealing elements on the piston in axially spaced-apart relationship so as to embrace the adapter, wherein the sealing elements form together a double-sided lip seal. This arrangement is especially compact, and the double-sided lip seal attains superior dynamic sealing properties.
According to another feature of the present invention, an annular restraining ring is provided on the piston for realizing a form-fitting securement of the sealing element on the piston while still allowing a relative rotation between these components.